This invention relates to a process for the preparation of hydrolytically stable binders, in a specific aspect to the preparation of hydrolytically stable carboxylated amide binders wherein the carboxyl groups are linked to the binder via an amide group, to coating compositions containing said hydrolytically stable binders, and to their application by electrodeposition or by conventional application techniques.
Although the use of water-soluble or water-reducible binders is increasing, one of the problems frequently encountered with aqueous coating compositions based on such binders is insufficient hydrolytic stability. This phenomenon is particularly observed with anionic water-reducible binders, i.e., those binders which derive their ultimate water-reducibility from the presence of a sufficient number of built-in carboxyl groups. The insufficient hydrolytic stability of anionic binders in aqueous media is generally related to the presence of hydrolytically unstable ester linkages, which are frequently introduced simultaneously with the introduction of the carboxyl groups.
One of many methods proposed to prepare ester linkage-free anionic binders has been described in British Patent Application GB No. 2,037,291A. In this method, an essentially epoxy-free, amino group-containing binder is reacted with a cyclic anhydride of a dibasic carboxylic acid to provide after a ring opening reaction a carboxylated amide binder, which upon neutralization with a suitable base becomes water-reducible.
Although the described method claims the synthesis of a new class of ester linkage-free water-reducible binders, there are problems associated with the method. One problem related to the use of cyclic anhydrides is the poor solubility of these generally solid materials in the reaction medium. Furthermore such anhydrides may contain, depending on the degree of purity, a certain amount of free acid. Both unreacted anhydride and free acid are very difficult if not impossible to remove from the rather viscous reaction medium. When such anhydrides are employed in the preparation of water-borne binders, the presence of unconverted anhydride or free acid in the ultimate coating composition can have a negative effect on the electrodeposition characteristics.
It is therefore an object of the present invention to develop a method for the preparation of carboxylated amide binders which does not involve the use of a cyclic anhydride.